1. Technical Field
The present invention relates to a radio transmitting apparatus and a radio receiving apparatus used in a digital radio communications system, and a method for them.
2. Background Art
In a typical digital radio communications system using a multicarrier communication scheme, for when communicating using a data channel for transmitting voice data and/or image data and a control channel for controlling a communication station at another end and a communication state, a method where a small number of subcarriers are assigned to the control channel while multiple subcarriers are assigned to the data channel in order to minimize power consumption in a mobile station has been proposed (Laid-Open Japanese Patent Publication No. 2001-274767; Laid-Open Japanese Patent Publication No. 2001-285927).
In this method, a receiving apparatus performs A/D conversion on the control channel at a relatively low sampling rate in order to receive the control channel of narrowband comprising a small number of subcarriers, and in response to having received the control channel, the sampling rate of A/D conversion for the received signal is taken to be high to prepare for receiving the data channel of wideband comprising multiple subcarriers.
However, in this conventional receiving apparatus, because the center frequency of a data channel comprising a plurality of subcarriers 2 is different from the center frequency of a control channel 6 as shown in FIG. 1, the frequency of a local signal for down converting the subcarriers of the control channel 6 needs to be changed in order to switch from the receiving of the control channel 6 to that of the data channel 4 as shown in FIG. 2.
Here, the local signal is a signal whose frequency is set to the center frequency of a transmit frequency band on the transmitter side and by which a D/A converted, transmitting signal is multiplied to up convert the transmitting signal. On the receiver side, the signal received via an antenna is multiplied by a local signal to down convert the received signal.
Therefore, when the center frequency of the control channel 6 is different from the center frequency of the subcarriers of the data channel 4, the local signal's frequency needs to be changed to the center frequency of the data channel 4 after receiving the control channel 6, in order to receive the data channel 4. Until a PLL (Phased Locked Loop) circuit generating the local signal becomes stable in changing the local signal's frequency, it is difficult to switch from the receiving of the control channel 6 to that of the data channel 4, thus having prevented the speeding up of switching between the control channel 6 and the data channel 4.